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Nail Length Calculator

Recommends a nail length for the thickness being fixed using the three-times rule, and shows the penetration into the base timber.

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Enter your figures and press Calculate.
In short Nail Length Calculator is a free online tool on inputtesthub.com that recommends a nail length for the thickness being fixed using the three-times rule, and shows the penetration into the base timber. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.

How to use the Nail Length Calculator

  1. Enter your figures in the input fields.
  2. The result updates as you type, together with the full working.
  3. Check the breakdown table to see exactly how the number was reached.

About the Nail Length Calculator

The Nail Length Calculator recommends a nail length for the thickness being fixed using the three-times rule, and shows the penetration into the base timber. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.

What people use it for

  • Picking a nail for 18 mm boarding.
  • Checking penetration is enough for a fixing.
  • Choosing between 50 mm and 65 mm nails.

Details

Rule of thumb3 × thickness
Penetration target2 × thickness
18 mm board50 mm nail
25 mm board75 mm nail

The maths behind it

This tool uses Nail length ≈ 3 × the thickness of the piece being fixed, so about two-thirds of the nail ends up in the base timber. Rounded to the nearest stock length. Every intermediate value is shown on screen, so the result can be checked by hand.

Near an end or an edge, drill a pilot at about 80% of the nail diameter. Hardwood and thin sections split along the grain, and a split has no holding power even though the nail looks fine from outside.

Frequently asked questions

Do I need an account to use the Nail Length Calculator?
No charge and no sign-up. There is no free tier to run out of because there is no paid tier — inputtesthub.com is funded by advertising rather than by metering the tool.
Can you see what I put into this tool?
No. The Nail Length Calculator runs entirely in your browser using JavaScript. Because the work happens locally, the tool also keeps working if your connection drops, and closing the tab is all it takes to discard everything.
Can I check the maths myself?
It uses Nail length ≈ 3 × the thickness of the piece being fixed, so about two-thirds of the nail ends up in the base timber. Rounded to the nearest stock length. The full working is printed under the result, so you can follow every step rather than trusting a black box.
When would I actually need the Nail Length Calculator?
In practice it gets used for three things: picking a nail for 18 mm boarding. Checking penetration is enough for a fixing. Choosing between 50 mm and 65 mm nails. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Any caveats I should know about?
Near an end or an edge, drill a pilot at about 80% of the nail diameter. Hardwood and thin sections split along the grain, and a split has no holding power even though the nail looks fine from outside.
What are the reference figures here?
Rule of thumb is 3 × thickness. Penetration target is 2 × thickness. 18 mm board is 50 mm nail. These are printed on the page itself, so you do not have to take them on trust.
Is the number rounded?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.

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